Process for the purification of light paraffinic petroleum distillates

ABSTRACT

A CONTINUOUS PROCESS FOR THE PURIFICATION OF LIGHT PARAFFINIC FRACTION BOILING IN THE RANGE OF 25* TO 100*C. AND CONTAINING AS MAJOR PORTION STRAIGHT CHAIN PARAFFINS, BRANCHED PARAFFINS AND CYCLOPARAFFINS AND AS A MINOR PORTION AN IMPURITY SELECTED FROM THE GROUP CONSISTING OF AROMATIC HYDROCARBONS, SULPHUR-CONTAINING COMPOUNDS, NITROGEN-CONTAINING COMPOUNDS, AND OTHER NON--HYDROCARBON IMPURITIES, AND MIXTURES THEREOF, THE SULPHUR CONTENT BEING NO GREATER 500 P.P.M., SAID PROCESS COMPRISING: HYDROGENATION SAID LIGHT PARAFFINIC FRACTION AT A TEMPERATURE IN THE RANGE OF 150 TO 375*C., UNDER A PARTIAL PRESSURE OF HYDROGEN OF 10 TO 100 KG./CM.2, AT A S VELOCITY OF 0.5 TO 20 V./V./H. AND A HYDROGEN TO HYDROCARBON RATIO OF 100 TO 3,000 NM.3/M.3, IN THE PRESENCE OF A HYDROGENATING CATALYST CONTAINING BETWEEN 0.1 AND 2% OF PLATINUM ON A SILICA-ALUMINA CARRIER, SILICA BEING IN MAJOR QUANTITY, AND AT LEAST 30% OF THE PLATINUM BEING DISTRIBUTED ON THE CATALYST SURFACE WHICH IS ACCESSIBLE TO THE REACTANTS, FRACTIONATING THE PRODUCTS OF THE HYDROGENATION TO SEPARATE THE HYDROGENATED SULPHUR- AND NITROGEN-CONTAINING COMPOUNDS AND TO RECOVER A PARAFFINIC FRACTION SUBSTANTIALLY CONTAINING ONLY PARAFFINIC HYDROCARBONS WITH AN INCREASED CONTENT OF CYCLOPARAFFINS.

99,431 i ept. 019 23 02, 23/04 v ABSTRACT; OF THEDISCLOSURE A continuousprocess for the' purification of light paraflinic fraction boiling inthe range of 25 to 100 C. and containing as major portion straight chainparaffins, branched parafiins and cycloparaffins and as a minorportion-an impurity selected from the group consisting of aromatichydrocarbons, sulphur-containing compounds, nitrogen-containingcompounds,-and other non-hydrocarbon impurities, and mixtures-thereof,the sulphur content being no greater than 500 p.p.m.,;said process comprn hydrogenating said light parafiinic fraction at a temperature' in" therange fof 1 50 to 375 C., under a partial fpress'ureofhydrogen of to 100kg./cm. at a space i velocity' of 0.5t0 2O v'./v.'/'h. and a hydrogen tohydrocarbon ratio of100to3l000 Nmfi/mfi, in the presence or a'hydrogenating catalyst containing between 0.1 and i 2% of platinum onasilica-alumina carrier, silica being "iii'major' quantity, and at least30% of the platinum being distributed on the catalyst surface which isaccessible to' the-reactants, fractionating the products of thehydrogenation to sepa'rate'the hydrogenated sulphurandnitrogen-containjing compounds and to recover 'aparaflinic fractionsubstantially containingonly paraflinic hydrocarbons with '5 anincreased content of cycloparaflins.

, This invention relates to a process for the purification bepractically free from aromatic hydrocarbons and other impurities, namelys'ulphur containing compounds, In par- Iticular, the use" of unleadedgasoline results in an increasing demand for branched paraffinichydrocarbons, some of these compounds having a. particularly high octanenumber. Therefore, it is desirable to isomerizethe straight chainparafiinsagf these light parafiinic fractions or, at

1 least, to increase the content of branched parafiins in thesefractions. However, with many isomerization processes,

the content of aromatic hydrocarbons and 'sulphur-con- J tainingcompounds in the feeds isomeri zed is of great importance. Many isomerization' catalyst'sare very sensitive to these impurities, must, be,removed as; completely a s .pos'sih1e... '1' =5l .ovrn methods forrefininglight parafiinic hydrocarbons generally require specialtechniques, such as a two-step pretreatment comprising the States Patent0 3,827,974 Patented Aug. 6, 1974 hydrodesulphurization of the feed andthen the hydrogenation of any unsaturated hydrocarbons. Such processingis expensive.

An object of the present invention is to provide a process for thetreatment of light parafiinic fractions in order to removesimultaneously the aromatic hydrocarbons and the sulphur-containingcompounds. Another object of this invention is to provide a processwhereby this treatment is performed with reduced investments andoperating costs.

According to the present invention, there is provided a continuousprocess for the purification of a light parafiinic fraction boiling inthe range of 25 to C. and containing straight-chain parafiins, branchedparaffins and cycloparafiins with a lower content of aromatichydrocarbons, sulphur-containing compounds, nitrogen-containingcompounds and other impurities, the sulphur content being preferably notgreater than 500 ppm, said process comprising subjecting said parafiinicfraction to hydrogenation at a temperature in the range of to 375 C.,under a partial pressure of hydrogen of 10 to 100 kg./cm. at a spacevelocity of 0.5 to 20 v./v./h. and a hydrogen to hydrocarbon ratio of100 to 3.000 Nmfi/mfi, in the presence of a hydrogenating catalystcontaining between 0.1 and 2% by weight of platinum on a silicaaluminacarriers, silica being in major quantity, and at least 30% of theplatinum being distributed on the catalyst surface which is accessibleto the reactants,

fractionating the products of said hydrogenation to separate thehydrogenated sulphurand nitrogen-containing compounds and to recover aparafiinic fraction substantially containing only parafiinichydrocarbons with an increased content of cycloparafiins.

Several methods are known for the hydrogenation of petroleum fractionscontaining aromatic hydrocarbons, particularly for the hydrogenation ofkerosene and fuels, in order to improve their combustion value. Butthese methods most frequently relate to the treatment of fractionsboiling in the range of 60 to 350 C., to reduce their content ofaromatic hydrocarbons. However, the hydrogenation yield is generally nothigher than 98%, this yield being nevertheless more than enough toincrease the combustion value of these fractions.

By contrast, the process of the present invention is applied to lighterfractions and it must perform to practically completely remove thearomatic hydrocarbons and also the sulphur-containing compounds.Therefore the present process differs from the prior processes in that aspecific feed is treated in the presence of a specific catalyst in orderto obtain a product which must have specified properties.

The feed to the present process mainly comprises parafiinichydrocarbonscontaining from 5 to 7 carbon atoms inclusive, more generally 5 to 6carbon atoms, the amount of straight-chain paraffins being at least 30%and more often at least 50%.by weight. The feed contains also b ranchedparaflins and cycloparatfins. The amount of aromatic hydrocarbons, moreparticularly benzene and toluene, may be present within wide limits, butgenerally is present in an amount within the range of 0.1 and 10% byweight. v

.The amount of sulphur-containing compounds present in the feed to theprocess of the presentin-vention genv ,erally is between about 50 and500 ppm. If ,the amount -.is higher, the parafilnic fraction isadvantageously desulphurized prior to use in the present process. Thisdesulphurization treatment need not be directed in such a manner'as tosecure a complete removal of the sulphurcontaining compoundsas requiredwith prior processes.

A caustic wash will likely suffice. Another means of suchdesulphurization comprises subjecting said petroleum cut to a sweeteningtreatment to convert mercaptans contained in said out into heavierdisulphides so that a subsequent distillation gives a parafiinicfraction with a low sulphur content. The object of these desulphurizingtreatments is only to reduce the sulphur content of the paraflinicfraction to a level lower than about 500 ppm. The prior processesgenerally require a complete removal of the sulphur-containing compoundsbefore hydrogenationof the parafiinic fraction and therefore, are moreexpensive.

In accordance with the process of the present invention, the lightparaflinie fraction is hydrogenated under the above specifiedconditions, preferably, however, hydrogenation is carried out at atemperature within the range of 175 to 325 C., under a partial pressureof hydrogen of between about to 75 kg/cm. and with a space velocity from0.5 to 20. The optimum conditions depend not only On the composition ofthe parafiinic fraction and on its content of aromatic hydrocarbons,cycloparaffinic hydrocarbons and sulphur-containing compounds, but theydepend also on each other. Therefore, the temperature to be applied fora substantially complete removal of the aromatic hydrocarbons varies asa function of the sulphur content of the feed, the other conditionsremaining equal. Also, the temperature may be lowered when the pressureis higher. Determination of such optimum value within the above definedranges may readily be made by those skilled in the art.

In order to economically perform, a substantially complete hydrogenationof the aromatic hydrocarbons and of the sulphur-containing compounds,the hydrogenating catalyst will contain from 0.1 to 2% by weight ofplatinum on a silica-alumina carrier, where silica is in major quantityand where at least 30% of the platinum is distributed on the catalystsurface which is accessible to the reactants. Indeed, it has been foundthat with catalysts, whose carrier does not contain .a greater amount ofsilica than alumina or whose platinum is poorly distributed onthesurface, the degree of aromatic hydrogenation is lower under theconditions which prevail than with the catalyst utilized in the presentinvention.

Many processes are known for the manufacture of catalysts containingplatinum on a carrier. However, a convenient process for preparing thepresent catalyst comprises impregnating the silica-alumina carrier witha solution of a platinum copound, usually chloroplatinic acid and thenevaporating the solution and thereafter drying and calcinating thecatalyst at about 400 C. Im-

wpregnation of the carrier may be performed with solutions of watersoluble platinum salts, but with the use of solutions having a pH fromabout 1 to 7. An extended drying and a moderate calcination of thecatalyst are particularly suitable. Under these conditions, at least 30%of the platinum is distributed on the total surface of the catalystwhich is accessible to the reactants. Determination of this percentageof platinum on the surface is carried out by using described methods,e.g. the method of D. E. Mears and R. C. Hansford, in Journal ofCatalysis, vol. 9, pp. 125-127 (1967).

In order to illustrate the effect of the catalyst composition,comparative hydrogenation tests were carried out on a light paraflinicfraction having the following composition:

Percent (by weight) n-Pentane 61.0 Iso-pentane 13.8 Cy'clopentane 4.72.2-dimethylbutane 0.6 2.3-dimethylbutane 0.4 Z-methylpentane I I 3.53-rnethylpentane 3.3 n-Hexane 8.2 Methylcyclopentane 3.0

Percent (by weight) Cyclohexane V 0.7 Benzene Sulphur, 200 ppm.

The operating conditions for hydrogenation were the following:

The catalyst used in each test consisted of platinum on a silica-aluminacarrier. However, the ratios of silica, alumina and platinum wereditferent in each test. The catalyst compositions and the results ofthese tests are summarized in Table I.

TABLE I Weight percent sum in Pt 0! On Residual Residual s Test numbercarrier catalyst surface benzene (p.p.rn.)

16. 2 0. 73 v 52 72 2 68. 7 0. 74 61. 0. 3 1 as. o o. as 42 o. 4 1 83. 90. 76 15 50 8 68. 7 0. 08 54 62 v 13 Results presented in the aboveTable I clearly demonstrate that the catalyst must fulfill all of theabove specified conditions relating to platinum content, the platinumdistribution on the surface of the catalyst and composition of thecarrier, in order to obtain a substantially complete aromatichydrogenation of the light parafiinic fraction. Tests 2 and 3 show thatthe hydrogenation yield is higher than 99.5%. 'By contrast, a catalystcarrier which has a lower content in silica (test 1) does not perform anadequate hydrogenation, although the content and the distribution of theplatinum are substantially the same as those of the catalysts of tests 2and 3. The same appears with a catalyst having a poor distribution ofthe platinum (test 4) or with a catalyst having a low content inplatinum (test 5).

Other hydrogenation tests were carried out in order to show the effectof temperature on the removal of aromatic hydrocarbons. In these tests,the catalyst contained 0.72% by weight of platinum on a silica-aluminacarrier with 03.8% of silica. The amount of platinum distributed on theaccessible surface of this catalyst was 43%.

The composition of the used light paraflinic'frac'tion hydrogenated wasas follows:

' Percent by weight 92 The sulphur content was varied during some of thetests by adding dimethyldisulfide. i

The operating conditions for hydrogenationwere as follows:

partial pressure of hydrogen: 50 lrg./crn.-' space velocity: 2 v./v./h.hydrogenzfeed ratio: 500 Nmfi/mfi,

The sulphur content of the feed'and the temperature employed in eachtest and the results are summarized in Table II.

TABLE II Residual Temperabenzene S content of feed ture C.) (p.p.m.)

50 .m 250 45 p p 270 7 290 5. 5 310 9 100 .m 250 200 p p 270 35 290 18310 12 330 30 TABLE III Residual Partial pressure of Temperabenzenehydrogen (kg/cm!) ture C.) (p.p.m.)

During the hydrogenation of the aromatic hydrocarbons contained in thelight par-aflin-ic fraction in accordance with the present invention,the sulphur-containing compounds are converted into H -S which is easilyremoved by distillation. Isomerization of some paraffinic compoundsoccurs also during this hydrogenating treatment. These advantages ofthis invention are shown by the results presented in Table IV of a testfor the hydrogenation of a light parafiinic fraction under the followingconditions:

temperature: 330 C.

partial pressure of hydrogen: 50 kg./cm. space velocity: 2 v./v./h.hydrogen/hydrocarbons ratio: 500 Nm. /m.

The hydrogenated products were distilled to remove H S formed during thehydrogenation.

TABLE IV Hydrogen- Compounds Feed ated product n-Pentane, percent by wt95. 2 86. 1 Iso-pentane 1 9. 1 Dimethylbutanes 5 5 MethylcyclopentaneTraces 3. 2 Cyclohexane 3. 3 1. 1 Benzene, p.p.m 9 31 Sulphur, p.p.m 1001 1 Cr 9,000 p.p.m.

The above mentioned tests show that the aromatic hydrocarbons of a lightparaffinic fraction may befhydrogenated with a practically quantitativeyield; Thebest results are obtained when the sulphur content of the feedis not higher than p.p.m. In such instance, the hydrogenation preferablyis carried out at a temperature between 275 and 330 C. and under apartial pressure of hydrogen varying between about 50 and 70 kg./ cm.and with the hydrogenation yield higher than 99.5%. Thesulphur-containing compounds are simultaneously converted into His,which is easily distilled off, so that the end product is practicallysulphur-free.

The results of Table IV show also that during this hydrogenation of thelight paraffinic hydrocarbon, an isomerization occurs with a yield whichis about 12% under the conditions employed in the example. This yieldmay be increased by operating at a higher temperature. However, thetemperature is kept within the above stated limits in order to secure apractically complete hydrogenation of the aromatic hydrocarbons.

For carryin out the subsequent isomerization of the paraffinic fraction,it is essential that the hydrogenated fraction does not contain morethan 100 p.p.m. of residual aromatics and not more than 30 p.p.m. ofsulphur, in order to prevent rapid deactivation of the isomerizationcatalyst. These conditions are easily fulfilled by applying the processof this invention.

From the foregoing description, one skilled in the .art can easilyselect the optimum operating conditions which are the most convenientfor a specific feed for the purpose of reducing the aromatic contentbelow about 10 p.p.m. and the sulphur content below about 5 p.p.m.

What is claimed is:

1. A continuous process for the purification of light paraffinicfraction boiling in the range of 25 to 100 C. and containing as a majorportion straight chain parafiins, branched parafiins and cycloparaflinsand as a minor portion an impurity selected from the group consisting ofaromatic hydrocarbons, sulphur-containing compounds, nitrogen-containingcompounds, and other non-hydrocarbon impurities, and mixtures thereof,the aromatic hydrocarbon content being between 0.1 and 10% by weight thesulphur content being no greater than 500 p.p.m. said processcomprising:

hydrogenating said light paraflinic fraction at a temperature in therange of to 375 C., under a partial pressure of hydrogen of 10 to 100kg./cm. at a space velocity of 0.5 to 20 v./v./h. and a hydrogen tohydrocarbon ratio of 100 to 3.000 Nm. /m. ,f in the presence of ahydrogenating catalyst consisting essentially of between 0.1 and 2% ofplatinum on a silica-alumina carrier, silica being in major quantity,and at least 30% of the platinum being distributed on the catalystsurface which is accessible to the reactants, fractionating the productsof the hydrogenation to separate the hydrogenated sulfurandnitrogen-conta" ining compounds and to recover a paraflinic fractionsubstantially containing only parafiinic hydrocarbons with an increasedcontent of cycloparatllns, less than 100 p.p.m. aromatic hydrocarbonsand less than 30 p.p.m. sulphur.

2. The process of claim 1 wherein the hydrogenation reaction is carriedout at a temperature of between and 325 C., under a partial pressure ofhydrogen ofbetween 15 and 75 kg./cm. and with a space velocitYof 0.5 to20 v./v./h.

3. The process of Claim 1 wherein said light paratfinic fractioncomprises one containing paraffinic hydrocarbons of 5 to 7 carbon atoms.

4. The process of Claim 1 wherein said light paraffinic fractioncontains no greater than 10% aromatic hydrocarbons by weight.

7 5. The process of Claim 1 wherein said light paraifinic fractioneontains at least 30% straight chain paraffin hyclrdcarbons.

References Cited UNITED STATES PATENTS 2,911,357 11/1959 Myers et a1.208138 Re.--26,883 5/1970 Marechal et al. 208143 3,203,891 8/1965 Holden2082l6 8 OTHER REFERENCES Mears et al., The Stoichiometry for HydrogenTitration of Oxygen on Supported Platinum, Journal of Catalysis, 9,1967, p. 125.

DELBERT E. GANTZ, Primary Examiner S. L. BERGER, Assistant Examiner U.S.Cl. X.R.

